This invention relates generally to a cryostat, and more particularly to a MRI cryostat that intercepts in-leakage of thermal energy at an intermediate temperature between cryostat load temperature and ambient temperature by using a closed throttle cycle refrigeration system in conjunction with a conventional open cycle cryogenic refrigeration system.
The desire to reduce health care costs has resulted in studies to reduce the costs of MRI systems. The basic problem in all cryostats is removal of load generated heat and/or heat which leaks in from the higher ambient temperature.
In a cryostat for magnetic resonance imaging (MRI), which contains a superconducting magnet, the magnet operates in a bath of liquid helium that vents to ambient in gaseous form as it absorbs heat. Thereby, the magnet may be maintained at a temperature in the order of 4K. To reduce the boil-off of liquid helium, present MRI cryostats typically use two-stage GM cycle refrigerators to cool shields and to intercept heat leakage at temperatures of about 60K and 15K. This construction has reduced liquid helium boil-off rate to a point where refill of the cryostat is necessary only about once a year. U.S. Pat. No. 4,606,201 describes a GM/JT (Joule-Thomson) refrigerator that mounts in a neck tube of a helium cryostat, intercepts heat leakage at two temperature levels above 4K, and recondenses helium at 4K. In many applications, it has been found economical to omit the 4K JT cold stage and use two stages of cooling that are available from a GM expander.
Use of the two-stage GM refrigerator substantially reduces consumption of helium through boil-off. However, the refrigeration system is both complex and expensive and draws high electrical power. Additionally, noise, vibration and disturbance of the magnetic field due to the use of the GM refrigerator weigh against usage of such systems in many applications.
What is needed is a MRI cryostat construction that reduces boil-off of liquid helium and uses a closed cycle refrigeration system having relatively low power consumption and operating without introduction of noise and vibration into the cryogenic and magnetic systems.